United States
Environmental Protection
Agency
Hazardous Waste Engineering
Research Laboratory
Cincinnati OH 45268
Research and Development
EPA/600/82-86/110 Apr. 1988
&EPA Project Summairy
Evaluation of Municipal Solid
Waste Landfill Cover Designs
John R. Dwyer, John C. Walton,
Wendy E. Greenberg, and Ronald Clark
The HELP (Hydrologic Evaluation of
Landfill Performance) Model was used
to evaluate the hydrologic behavior of a
series of one-, two-, and three-layer
cover designs for municipal solid waste
landfills. The specific landfill cover
designs studied were chosen to isolate
the effects of features such as surface
vegetation, thickness, soil type and
hydraulic conductivity of the layers on
the average annual runoff, cover perco-
lation, evapotranspiration, and lateral
drainage. The results of the evaluations
are presented in numerous bar charts to
supplement the tables and schematics.
Soil hydraulic conductivity was one
of the most important design features
in controlling cover percolation for all
covers tested.
Minimal cover percolation and runoff
were the two main criteria used to
select the four best cover designs,
which are described under Results and
Discussion. The full report also includes
a brief discussion of four other aspects
of landfill cover design: animal and
vector control, subsidence, gas control,
and cost.
This Project Summary was developed
by EPA's Hazardous Waste Engineering
Research Laboratory, Cincinnati, OH, to
announce key findings of the research
project that Is fully documented In a
separate report of the same title (see
Project Report ordering Information at
back).
Introduction
A primary concern associated with
municipal solid waste landfill construction
is ground-water pollution caused by pre-
cipitation becoming contaminated as it
moves through the wastes in the landfill
and percolates into the ground-water flow
system. An important objective of landfill
design is to minimize ground-water pol-
lution by controlling the water percolating
in and subsequently out into the ground
water. To help landfill designers meet
this objective, the U.S. Environmental
Protection Agency developed the HELP
Model, a computer model that simulates
water movement in and out of a landfill
under a variety of landfill cover designs
and climatic conditions. The Model uses
a water balance approach.
Atypical landfill can be subdivided into
three basic components: the landfill cover
or cap, the waste layer, and the liner. The
landfill cover provides the first line of
protection for the landfill. A well-designed
cover prevents water from coming into
contact with the waste layer, and mini-
mizes the risk of subsequent ground-
water contamination. The waste layer
contains the landfill refuse and the liner
is the bottom barrier which is designed to
protect the ground water from con-
tamination.
Procedure
The HELP Model simulates a multi-
layered landfill and provides a variety of
options for selecting the number and
type of layers in the landfill, the hydrologic
properties of the layers, surface vegeta-
tion, and other features of the design, it
then simulates water movement through
the landfill based on temperature and
precipitation data from one of several
dozen U.S. cities. The HELP Model cal-
culates the components of the water
balance for a specified landfill design,
including, runoff, evapotranspiration,
lateral drainage and percolation. These
parameters can be average annual values,
monthly values, or daily values, depend-
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ing on the HELP Model option selected.
The results of the model provide the user
with information on the effectiveness of
landfill design components for controlling
water movement through the landfill.
Results and Discussion
Minimal cover percolation and runoff
were the two main criteria used in select-
ing the best cover designs. Four cover
designs met these objectives and are
described below.
(1) The one -layer covers showed re-
duced percolation as the cover soil
ranged from coarse-textured (sandy)
soils to fine-textured (clayey) soils.
Percolation for one layer covers
using a clayloam soil was measured
as less than 5 inches in Denver,
San Francisco, Corpus Christi, and
Milwaukee, where the average
annual precipitation is less than 33
inches.
(2) Two-layer covers with a 6-inch
gravel layer over a clay barrier,
percolated less than 1.7 inches for
all 10 cities tested. Runoff was
relatively high ranging from 1.6 to
24 inches for the 10 cities.
(3) Percolation was slightly reduced for
two-layer covers consisting of a
topsoil layer over a clay barrier as
the soil texture of the topsoil layer
went from coarse to fine. Cover
percolation increased by 0.6 to 0.8
inch when the thickness of the clay
barrier layer was reduced from 24
to 12 inches. Cover percolation was
reduced by an approximately factor
of 10 when the hydraulic con-
ductivity of the barrier layer reduced
by a factor of 10. The HELP Model
results indicate that these covers
experienced saturation of the top-
soil layer during the 10 to 20 year
periods recorded for the 10 cities
tested. This saturation could be
harmful to vegetation and could
result in excessive surface runoff,
depending on the frequency and
persistence of saturation under a
particular climatic regime. Further
investigation is necessary to deter-
mine the suitability of these cover
types for a given climate.
(4) Three-layer covers consisting of a
topsoil layer, a drainage layer, and
a clay barrier reduced percolation
by 0.25 to 0.55 inch when clay
loam instead of sandy loam was
used in the topsoil layer. Reducing
the hydraulic conductivity of the
drainage layer by a factor of 8.4
resulted in a reduction in cover
percolation ranging from 0.5 to 0.9
inch for the 10 cities. Results for
Lexington, Kentucky indicated that
cover percolation was reduced by 3
to 5 percent for every 1 percent
increase in the slope of the drainage
layer and was reduced by 3 to 9
percent for every 25-foot decrease
in drainage spacing, as the spacing
ranged from 200 to 25 feet. Reduc-
ing the hydraulic conductivity of
the barrier layer by a factor of 10
significantly reduces cover percola-
tion. The amount of the reduction
depends on the initial value of the
hydraulic conductivity and the
precipitation at the site.
Conclusions and ~
Recommendations
The findings from this study provide
useful guidelines for designing a cover
for a specific landfill site in a particular
climatic regime. However, the results
presented in this report describing the
effects of various design parameters on
the hydrologic performance of landfill
covers are predictions based on the HELP
Model and should not be interpreted as
absolute predictions of landfill behavior.
The results, which compare the hydrologic
performance of various types of one-,
two-, and three- layer covers, are intended
to help determine the relative importance
of various design parameters in terms of
their effect on hydrologic performance
for a range of climates.
Because this report analyzes the hydro-
logic behavior of landfill cover designs
solely in terms of average annual values,
the performance of a cover design during
a storm, although important, was beyond
the scope of this analysis. Although the
full report deals primarily with annual
averages, the HELP Model can be used to
analyze the daily water balance of a cover
design, employing user-supplied daily
precipitation data.
The hydraulic conductivity of one-layer
cover designs, as determined by soil type
and degree of compaction, is the most
important design feature in controlling
cover percolation. For two- and three-
layer covers containing a barrier layer,
the hydraulic conductivity of the barrier is
the single most important parameter in
controlling cover percolation. Cover
percolation can be specified by adjusting
the hydraulic conductivity of the barrier
layer for most of the two- and three-
layer cover designs examined in this
study.
Three-layer covers that include a
drainage layer generally result in lower
cover percolation than one- or two-layer
covers. Results of this study also show
that the hydrologic behavior of the same
landfill cover design can vary significantly
in different climates.
In designing a landfill cover, perfor-
mance criteria should first be established.
These include the objectives of the design,
namely the maximum amounts of cover
percolation and runoff, and the acceptable
amount and frequency of vertical percola-
tion layer saturation. These objectives
are based on such factors as: the type of
waste to be stored in the landfill; the type
of liner beneath the waste layer; geologic
conditions in the vicinity; the storm
magnitude that the cover is intended to
withstand; the local climatic regime; the
potential threat of groundwater contami-
nation; proximity to aquifers, residential
areas, or wildlife habitat; and budget
constraints.
Animal and vector control, subsidence,
gas control, and cost are four other factors
considered in landfill cover design.
Animals burrowing at landfills can be
controlled by using a cover of dry, loose
gravel and sand to discourage tunneling.
Birds and insects can be controlled by
installing a thick, compacted, well-graded
surface layer.
Techniques for controlling subsidence
of landfill covers are still being developed,
but can be divided into two categories: (1)
those that control subsidence in the waste
layer and (2) those designed so the cover
layer features minimize the effects of
subsidence. Techniques used in the first
method increase the ratio of daily soil
cover to waste, and compaction. An
example of the second is to create a
stable foundation or at least a 24-inch
buffer layer of compacted sand or gravel
-between-the base of the- cover and the
waste layer.
Control of methane gas and volatile
toxic vapors at landfill sites is an im-
portant consideration. Methane control
techniques are designed to limit methane
production and to control lateral subsur-
face migration by venting the gas to the
atmosphere. With volatile toxicants, both
lateral migration and atmospheric emis-
sions must be controlled. Some of the
techniques for gas control are gas drain-
age layers with vents, vertical pipe vents,
trench vents, and induced gas extraction
systems. These techniques are discussed
briefly in the report. References describing
these techniques in more detail are listed
in the report bibliography.
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The major expense in landfill cover
construction is the material and instal-
lation costs for topsoil, sand, clay, and
synthetic liners. The report also gives
cost estimates for the four recommended
cover types.
The full report was submitted in ful-
fillment of Contract No. 68-03-3248,
Work Assignment No. 5 by Battelle
Columbus Laboratories under the spon-
sorship of the U.S. Environmental Protec-
tion Agency.
John R. Dwyer, John C. Walton, and Wendy E. Greenberg are with Battelle
Denver Operations, Denver, CO 80209; Ron Clark is with Battelle Columbus
Laboratories, Columbus, OH 43201.
Robert Landreth is the EPA Project Officer (see below).
The complete report, entitled "Evaluation of Municipal Solid Waste Landfill
Cover Designs," (Order No. PB 88-171 327'/AS; Cost: $19.95, subject to
change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Hazardous Waste Engineering Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
*U.S. GOVERNMENT PRINTING OFFICE: 1988—550-289/62071
3
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United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
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POSTAGE & FEES PAID
EPA
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EPA/600/S2-86/110
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